Machining metals and other target specimens using a laser beam generates a significant amount of molten debris. Most of the debris is ejected from the immediate region surrounding the laser cut, as the laser beam blasts through the target specimen material. Debris from the area within the width of the laser cut, or “kerf,” may be ejected by a high-pressure jet of cutting head gas flowing along the laser beam propagation axis and out of a nozzle through which the laser beam is focused. Thus, the laser beam propagates and the cutting head gas flows along a common axis. Remaining debris particles are ejected at high velocity (several km/sec) along trajectories perpendicular to the kerf, both axially of (i.e., normal to) the target surface and parallel to the target surface of the target specimen undergoing machining. The sizes of these particles range from sub-millimeter to sub-micron, and the particle temperatures are high, typically at least several hundred degrees centigrade. Without proper debris containment during the laser micromachining process, the laser system becomes polluted with axial debris and requires daily cleaning and maintenance. In addition, surface debris may block the laser beam cutting path, reducing ablation efficiency.
The current state of the art of debris management in semiconductor micromachining systems is highly dependent on the application. In some applications, such as for example, semiconductor wafer scribing, processing may be restricted to the wafer backside, thereby completely avoiding the target surface and steering clear of active layers of circuitry. Other applications address debris ejected through the underside of a target material undergoing laser micromachining, while the remainder of the debris on the target surface of the material is not managed or contained. Most laser micromachining systems are designed with proper covers and shields to protect sensitive subsystem components from vapor and molten deposits, but these shields intercept and trap only a small portion of the ejected material. Although they protect the micromachining equipment, the shields do not address quality assurance of the electronic parts being processed.
When drilling prescribed holes, a “sandwich” technique may be used that entails covering both surfaces of the target with a protective layer, drilling through both the protective coverings and the target material, and later peeling off the coverings and surface debris together (Tuan A. Mai, “Toward Debris-free Laser Micromachining,” Industrial Laser Solutions, 23:1, 2008). Another similar technique entails coating a surface with a benign protective layer (e.g., photoresist) that traps debris and can be dissolved after laser processing. Yet another technique entails cutting in the presence of a water spray or a water film bathing the target surface; however, the presence of liquid tends to result in mist or condensation affecting the laser optics (Sun and Longtin, “Ultrafast Laser Micromachining with a Liquid Film,” Proc. ICALEO, 2001).
Brushes have been used as debris management devices in related industries, such as printed circuit board (PCB) milling that uses end mills to drill macroscopic holes in a plastic PCB backplane to enable routing of the printed circuits. Some designs incorporate vacuum exhaust, but the systems currently implementing these designs do not fully encompass the cutting area. A considerable amount of material may, therefore, escape from the debris containment system. In the PCB milling application, an external vacuum hose may be attached to the back of the circuit board to enable intermittent application of vacuum pressure to remove the board material as it is drilled out. Alternatively, a brush may surround the drill bit, or “end mill,” and associated end mill spindle, and a brush housing that supports the brush may be equipped with a vacuum port to exhaust debris generated by drilling the board material. An example of such PCB milling equipment is a Final Touch 101 depaneling router system, available from Precision PCB Products of Irvine, Calif.